The present invention relates to an image processing apparatus and, more particularly, to one enabling estimation of a halftone image from a multicoded image.
Many of the outputting apparatuses, that is to say, indicators and printing apparatuses, in commercial use today are limited to reproduction only in white and black. It is known that a technique called "density pattern method" (luminance pattern method) or another called "dither method", both applicable by means of such conventional outputting apparatuses, is employed for a simulative effect in representing halftones. Both the density pattern method and the dither method are a kind of per area gradation method, consisting in changing the number of dots put in a unit area (matrix).
The density pattern method, as shown by FIG. 22-b, consists in recording, by using a threshold value matrix, in the part corresponding to a pixel of the original with a plurality of dots, whereas the dither method is, as shown by FIG. 22-a, to record with one dot in the part corresponding to a pixel of the original. Both of these methods give, as shown by the same illustrations, binary coded output data, which represent halftone images with two values "white" and "black" in a manner of simulation.
In such conversion, it is advantageous for a method to be capable of reverse conversion from halftone images simulatively represented by binary coded values to the original halftone images, i.e., input data as shown in FIG. 22, because such capability permits various modes of processing of the data and hence conversion of images in various ways. An image represented in a density pattern can be put back directly to its original halftone image if the array of the pattern levels is determined, but the resolution is low relative to the information content. Contrarily, a dither image, compared with a density pattern image, enables a high resolution relative to the information content, but it is difficult for a dither image to be put back to the original halftone image. Therefore, a dither method alone is not capable of image conversion to suit a variety of needs.